Abstract base class representing a storage format for a field or vector.
Format objects are responsible for writing and reading the low-level
representation of a field. It controls what kind/level of information to
store about the indexed fields.

Parameters:

field_boost – A constant boost factor to scale to the score
of all queries matching terms in this field.

Takes the text value to be indexed and yields a series of
(“tokentext”, frequency, weight, valuestring) tuples, where frequency
is the number of times “tokentext” appeared in the value, weight is the
weight (a float usually equal to frequency in the absence of per-term
boosts) and valuestring is encoded field-specific posting value for the
token. For example, in a Frequency format, the value string would be
the same as frequency; in a Positions format, the value string would
encode a list of token positions at which “tokentext” occured.